The invention relates to a method of purifying a hydrogenated gas containing carbon monoxide and hydrogen sulphide. It also relates to applications of the method.
Hydrogen is becoming increasingly important in the chemical, oil and petrochemical industries. It is used inter alia for synthesizing methanol, ammonia or synthetic natural gas. it is also increasingly used for refining crude oil, inter alia in hydrocracking and hydrodesulfurization processes which are being increasingly developed in order to make maximum use of the heavy fractions of crude oil.
Hydrogen can be obtained from gas mixtures coming from a wide variety of sources. It may, for example, be a by-product of catalytic reforming or steam-cracking processes. It may also be obtained by hydrocarbon decomposition processes, such as partial oxidation of feeds of substances varying from methane to very heavy hydrocarbon fractions, or steam reforming of light fractions.
The resulting gas mixtures, which contain hydrogen, may also contain hydrogen sulphide, carbon oxysulphide, carbon monoxide, carbon dioxide and light hydrocarbons such as methane. Usually one or more of these gases has to be eliminated before using the gas mixture containing hydrogen. This applies more particularly to gases such as carbon monoxide and hydrogen sulphide, which are both present in certain gas mixtures, for example those obtained by partial oxidation of hydrocarbons.
The term "hydrogenated gas" as used hereinafter denotes a gas mixture containing at least 30% by volume of hydrogen, the gas mixture also containing at least carbon monoxide and hydrogen sulphide; the term "desulphurized gas" denotes the hydrogenated gas after removal of hydrogen sulphide, and "purified gas" denotes the hydrogenated gas after removal of the carbon monoxide, hydrogen sulphide and carbon dioxide present initially in the hydrogenated gas or formed during the purification thereof, by conversion of carbon monoxide to carbon dioxide.
In order to eliminate carbon monoxide, hydrogenated gases are purified by various methods such as chemical conversion or extraction by a solvent.
Carbon monoxide can be catalytically converted to carbon dioxide by the reaction: EQU CO + H.sub.2 O .fwdarw. CO.sub.2 + H.sub.2
the reaction is performed in the presence of catalysts, the nature of which varies with the operating temperature.
The "high-temperature" catalyst comprises chromium oxide and iron oxide and can be used from approximately 340.degree. to approximately 600.degree. C. The catalyst, which is relatively insensitive to sulphur compounds, can reduce the carbon monoxide content of the purified gas to approximately 3.5% by volume.
"Low-temperature" catalysts, comprising oxides of zinc, copper, aluminum or chromium, can be used from approximately 180.degree. to approximately 380.degree. C. These catalysts are very sensitive to sulphur compounds. They can reduce the carbon monoxide content of the purified gas to approximately 0.2% by volume.
The "medium-temperature" catalyst can be used at temperatures from 280.degree. to 350.degree. C. It is made up of cobalt and molybdenum oxides. It requires the presence of hydrogen sulphide at a concentration of 800 to 1200 ppm. This catalyst, therefore, is not poisoned by hydrogen sulphide. It can reduce the content of the hydrogenated gas to 1.5% CO.
In the remainder of this specification "high-temperature", "low-temperature" and "medium-temperature" conversion units denote carbon monoxide conversion units using "high-temperature", "low-temperature" and "medium-temperature" catalysts respectively.
Another method of converting carbon monoxide in hydrogenated gas is the "methanation" method, which comprises converting carbon monoxide to methane in accordance with the reaction: EQU CO + 3 H.sub.2 .fwdarw. CH.sub.4 + H.sub.2 O
numerous catalysts have been used in this method. The most widely used have a relatively high nickel content, namely 25 to 30% by weight. These catalysts are easily poisoned by sulphur. Methanation is performed at a temperature of from 180.degree. to 440.degree. C. In view of the amount of hydrogen required for methanation (3 molecules of hydrogen per molecule of carbon monoxide) the gas to be purified should preferably contain not more than 0.5% by volume of carbon monoxide.
In accordance with this specification, a "methanation" unit is a unit for working the methanation process for converting carbon monoxide to methane.
Alternatively, carbon monoxide can be eliminated from the hydrogenated gas by absorption and formation of a complex. The absorbent used can be a solution of a copper and ammonium salt. Alternatively, an aromatic copper complex can be used as described in Informations Chimie No. 132 (May 1974), page 261.
Hydrogen sulphide can be eliminated from the hydrogenated gas by absorption processes at temperatures near or below ambient temperature, using solvents such as methanol, N-methylpyrrolidone or a mixture of sulpholane and an alkanolamine.
Next, hydrogen sulphide is restored by heating the solution, then converted to sulphur in a Claus unit, where hydrogen sulphide reacts with sulphur dioxide as follows: EQU 2 H.sub.2 S + SO.sub.2 .fwdarw. 3S + 2 H.sub.2 O
usually Claus units comprise three steps: Sulphur dioxide is produced by oxidation of hydrogen sulphide in a thermal step at a high temperature, of the order of 1400.degree. C; the Claus reaction occurs in the vapour phase in two catalytic steps in which the temperature is of the order of 200.degree. to 250.degree. C.
The gas collected at the outlet of the Claus unit still contains a low proportion of hydrogen sulphide and sulphur dioxide, which has to be eliminated to avoid atmospheric pollution when the gas is discharged to the atmosphere. Consequently, methods using a Claus unit have the disadvantage of requiring a "tail gas treatment" unit. More particularly, in addition to the last-mentioned disadvantage, considerable heat is consumed in the step for restoring hydrogen sulphide before the Claus unit.
Examples of the combined method of eliminating carbon monoxide and hydrogen sulphide from a hydrogenated gas are given in the paper presented by M. J. Milner and D. M. Jones at the 8th World Petroleum Congress (Moscow, 13-19 June 1971). This paper describes the treatment of hydrogenated gases obtained by partial oxidation of hydrocarbons; the hydrogenated gases are cooled, either by quenching or in a boiler, before elimination of carbon monoxide and hydrogen sulphide. Carbon monoxide is eliminated by catalytic conversion; hydrogen sulphide is eliminated by selective absorption followed by a Claus unit, and carbon dioxide is eliminated by selective absorption.